Depending upon the nature of a load that a refrigeration system is required to service, cooling load requirements imposed on the refrigeration system by the load can vary widely, such for example as when the refrigeration system serves the cooling requirements a frozen beverage product dispenser. In the case of a frozen beverage product dispenser, customer demand for beverages can vary from no drinks dispensed per minute to as many as 3 or 4 or more drinks served per minute. This volatile variation in customer demand results in a very broad range in cooling load requirements for a refrigeration system of a typical frozen product dispenser, for example as is shown by the chart of FIG. 9. As can be seen, depending upon ambient temperature and during periods when no product is being drawn, the maintenance cooling load of a frozen product dispenser can be as low as about 1500 Btu/hr. At the other extreme and during periods of high drink draw rates, for example when delivering drinks at the rate of 4×16 oz drinks per minute, cooling load requirements of a frozen product dispenser may be in excess of 18,000 Btu/hr. This represents about a 12:1 turndown ratio, which from an energy standpoint conventional refrigeration systems are not able to efficiently accommodate.
As is known, refrigeration systems of conventional frozen product dispensers utilize a compressor that delivers refrigerant through a condenser to one or more expansion valves, each of which delivers refrigerant to an associated evaporator coil that is heat transfer coupled to an associated beverage product freeze barrel in order to chill the barrel and at least partially freeze beverage product in the barrel. To accommodate various cooling load requirements of the barrels, the expansion valves are variably controlled to meter refrigerant at various flow rates to the evaporators. As load requirements of an evaporator coil change due to changing customer demands, the expansion valve supplying refrigerant to the evaporator changes to a more appropriate flow metering position. The objective is to adjust the expansion valve so as to match the cooling capability of the evaporator, based upon refrigerant flow to the evaporator, more closely to the dynamically changing cooling load requirements of the barrel being chilled by the evaporator. However, since customer demand for frozen beverage product can vary from no drinks served per minute to as many as 3 or 4 or more drinks served per minute, it is not practical to rely upon a typical control system loop using temperature sensors for feedback. Temperature sensors are too slow to respond to a need for chilling product, since they have a 1st order response time on the order of 5 seconds or more, and meanwhile there is a cooling load that is changing faster than the temperature sensors can respond to and the control system cannot keep up with the changing cooling load requirements. In addition, a fixed speed compressor, as is normally used for a frozen product dispenser, is not readily able to accommodate changes in cooling load requirements and is best suited to providing refrigerant flow at a certain constant rate, despite changes that may be occurring in the cooling load that could best be served by varying the flow rate of refrigerant from the compressor. Consequently, refrigeration system balance becomes disturbed as the expansion valves are frequently adjusted in an attempt to meet changing cooling load requirements, resulting in saturated evaporator temperatures dropping as cooling load requirements decrease, rising as cooling load requirements increase, and overall poor control over the temperature of the evaporator. In addition, because the compressor operates at a constant speed, when cooling load requirements decrease, cooling of beverage product in the barrel is quickly satisfied and the compressor must be frequently cycled off/on, resulting in increased stress of compressor components. As a result, where the compressor is not matched with the cooling load, during periods of low product demand the compressor will cycle on/off excessively and the system will operate less efficiently and use more energy than is required.